Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $244.43   12.308%   $274.5144   $274.51   $274.50   $274.50  
  Purchase #2 $138.26   7.665%   $148.8576   $148.86   $148.85   $148.90  
  Purchase #3 $73.68   6.163%   $78.2209   $78.22   $78.20   $78.20  
  Purchase #4 $189.46   11.789%   $211.7954   $211.80   $211.80   $211.80  
  Purchase #5 $38.39   4.828%   $40.2435   $40.24   $40.25   $40.20  
  Purchase #6 $101.67   7.738%   $109.5372   $109.54   $109.55   $109.50  
  Purchase #7 $123.79   8.441%   $134.2391   $134.24   $134.25   $134.20  
  7 purchase totals = $909.68       $997.4081   $997.41   $997.40   $997.30